Field of the Invention
The invention relates to an amplifier array with digitally adjustable total gain that is constructed of a multiplicity of switchable individual amplifiers, and to a receiver circuit, in particular a mobile radio receiver circuit, with such an amplifier array.
The receiver circuits of mobile radio handsets, for instance, must each be capable, depending on the distance from the base station, of processing variably high levels of a signal to be received, and a prevalent condition is that an output signal of the receiver should have a level that is as constant as possible. The output level should be constant not only over time but also over temperature, and should also have the greatest possible signal-to-noise ratio. Moreover, it should be as independent as possible from operating voltage fluctuations and from production variations. A typical receiver circuit is shown in a block circuit diagram in the paper entitled "An RF Front-End for Digital Mobile Radio" from IEEE 1990, Bipolar Circuits and Technology Meeting, pp. 244 to 247. There, an antenna is supplied through a switch and a first filter to a HF preamplifier that is constructed with a gain regulation input. Its output signal is delivered through a second filter to a first mixer. Its output signal, the intermediate frequency signal, is filtered through the use of a SAW filter and is delivered, through an intermediate frequency (IF) amplifier, to a second mixer, having an output signal which is then the base band signal and can be delivered, for instance, to an A/D converter. The IF amplifier also has an input for regulating its gain. Overall, by regulating the gain of the preamplifier and the IF amplifier, the level of the base band signal can be kept at a virtually constant value, even though the reception signal which is present at the antenna can have major differences in level. The amplifiers are regulated by providing that the resistance of a diode, connected parallel to the load resistor of a given amplifier, is varied through the use of a current source that triggers the diode. However, the disadvantage of that type of analog regulation is that the gain for a predetermined regulated voltage variable has excessive temperature drift, and that the magnitude of that drift depends on the gain value that has just been adjusted. Moreover, the input levels of the differential stages of those amplifiers are limited to 200 mVpp, and the linearity also varies with the status of regulation. Another disadvantage is that the input noise ratio cannot be adjusted to the lowest possible values.
The paper "A 100 MHz IF Amplifier/Quadrature Demodulator for GSM Cellular Radio Mobile Terminals", from IEEE 1990, Bipolar Circuits and Technology Meeting, pp. 248-251, discloses a digital adjustment of the amplifier values of the IF amplifier for a mobile radio receiver. To that end, four amplifiers are connected in series, and they can be switched for gains of between 0 and 24 dB, 0 and 12 dB, 0 and 6 dB, or 0 and 3 dB, respectively. Accordingly, gains between 0 and 45 dB can be established, in increments of 3 dB. However, the disadvantage of that known digital adjustment is that only the smallest possible increment size of 3 dB is provided. Moreover, for a steady increase or decrease in the total gain, individual amplifiers must be repeatedly turned on and off again, which worsens the linearity of the overall configuration.